The present invention relates to the field of electromagnetic energy interference cancellation.
Receivers, for example radio frequency (xe2x80x9cRFxe2x80x9d) receivers, are typically designed to operate in a given bandwidth (e.g, UHF or microwave bands) and may also be designed to operate over multiple bandwidths. Receivers may be designed to operate on signals occupying a subset of the receiver""s overall operating bandwidth. Such a subset may be called a xe2x80x9cchannel.xe2x80x9d The frequency at the center of a channel""s bandwidth may be called the xe2x80x9cchannel frequency.xe2x80x9d Electromagnetic signals from, for example, RF transmitters, may interfere with a receiver""s operation even if the electromagnetic signals"" frequency spectra do not substantially overlap the receiver""s channel frequency. Out-of-channel (also known as out-of-band) interference signals can create adverse effects in a receiver (e.g., desensitization, cross-modulation, and intermodulation). In environments such as airports, communications facilities, or mobile platforms, interference signals are of particular concern due to the proximity of co-located transmitters and receivers. It is desirable to suppress out-of-channel interference signals incident on a receiver""s input.
Tunable, or even fixed, preselector filters placed in series with and preceding a receiver may offer some suppression of out-of-channel signals (and therefore some reduction in adverse effects), however, such filters are typically costly. Preselector filters typically possess bandwidths that are substantially larger than the operating channel bandwidth, which is especially true for tunable preselector filters. Furthermore, such filters add weight and occupy physical space that may not be available in certain platforms. Additionally, preselector filters can introduce distortion by, for example, preselector filter group delay.
Devices known as cosite interference cancellers, typically used in environments having co-located transmitters and receivers, have been used to reduce the amount of interference signal that is incident on a receiver. However, known applications of cosite interference cancellers require that a reference signal, sampled from a source external to the cosite interference canceller itself, be applied to the cosite interference canceller""s circuitry. The reference signal is typically a sample of an interfering signal. This sample is subtracted from a composite signal; the composite signal contains both the desired and interference signals. U.S. Pat. No. 5,630,223 to Bahu et al. requires a plurality of filter circuits for receiving filtered transmission signals (i.e., samples of interference signals from transmitters). U.S. Pat. No. 5,428,831 to Monzello et al. discloses a reference coupler for sampling the transmitted signal of a co-located radio transmitter. These patents exemplify a class of devices that require a hardwired connection of a reference signal (i.e., the interfering source of electromagnetic energy) to the cosite interference canceller. U.S. Pat. No. 4,893,350 to Minamisono et al. discloses a cosite interference canceller that may be operated with interference sources that are not hardwired to the cosite interference canceller. Minamisono et al. discloses an auxiliary antenna positioned in the direction of the interference signal for collection of essentially only the interference signal. The output of the auxiliary antenna supplies the interference canceller with a reference signal used to cancel the interference signal from a composite signal collected by a main antenna. Minamisono et al. is representative of a class of cosite interference cancellers that use auxiliary antennas to collect reference signals.
The present invention relates to a method and apparatus for interference cancellation, where interference signals are derived from a composite signal, and where the composite signal contains both the desired and interference signals. The derived interference signal may be used as a reference signal for the interference canceller. A preferred embodiment does not require hardwiring or cooperation between it and an interfering electromagnetic energy source. It operates on a composite signal and may utilize a single input port for signal reception (i.e., a separate reference port is not required). No a priori knowledge of the interfering source""s frequency spectrum is required.
The preferred embodiment derives its own reference signal by suppressing the desired signal in a duplicate of the composite signal. Suppression of the desired signal is accomplished in a subcircuit of an interference canceller referred to hereinafter as a Reference Circuit. The Reference Circuit derives a likeness of the interference signal. The likeness of the interference signal may be processed so that it is substantially equal in amplitude and opposite in phase to its counterpart in the original composite signal. The composite signal and likeness of the interference signal may be summed, thereby substantially canceling the interfering signal from the composite signal. The resultant signal, having a substantially suppressed interference signal, may be applied to the input of a receiver. The Reference Circuit may be tunable.
The interference signal may be located close in frequency to the desired signal. The desired signal may have weaker signal strength than the interference signal. No knowledge of the interference signal is needed preferably, only knowledge of the desired signal""s center frequency is required. The preferred embodiment is useful to avoid desensitization of receivers through suppression of narrowband or wideband out-of-channel interference. The method and apparatus disclosed herein can be used in conjunction with various weight control algorithms (e.g., the Least Mean Square algorithm).
One object of the invention is to provide an improved interference canceller.
Another object of the invention is to provide an interference canceller that does not need a priori knowledge of an interfering signal""s electromagnetic spectral content.
Another object of the invention is to provide weak signal extraction by suppression of strong interference signals.
Another object of the invention is to provide a reference circuit to suppress desired spectral components from a composite signal containing both the desired and the interference spectral components, thereby leaving substantially only the interference spectral components.